I will be climbing Illimani in Bolivia this summer with Mountain Madness. We'll go up to around 21k feet. I have some questions about my layering and need some help. I've been up to around 18k feet before, but didn't really attempt to dress properly, just put a lot of clothes on! For this trip, I want to do it right. Here's the gear I have...

Base - Under Armor cold gear compression
Soft-shell - North Face Apex jacket
Insulating layer - Helly Hansen Odin Belay OR La Sportiva Pegasus
Hard-shell - nothing yet

Any thoughts on what I have and recommendations on what to change/add would be great!

I climbed Illimani in September 2009. That's late in the climbing season, which means it's supposed to be somewhat less cold. Mind you, Bolivia isn't all that far from the equator so I don't think it matters all that much.

I had a thermal base layer. Not sure anymore which, but it may very well have been one by Craft .
As a second layer I had a real cheap but warm fleece.
By far the most important piece of clothing to keep me warm was my down jacket, which was a RAB Neutrino Endurance.
I had a hard shell too, but I've forgotten which one. Chances of getting rain are real slim, so just about any hard shell will do.

With some minor variations, this is what I always bring. The main difference between 6000-ers in the Andes and, say, climbing in the European Alps, is that I rarely need a down jacket in Europe. But when it's really cold, that RAB jacket always keeps me warm.

Everything rgg said above, would add that one of the newer mid layer down jackets like the Arcteryx cerium lt will do unless you run cold. With a warm fleece underneath the light down will work as the outer layer for Bolivia but carry a light shell in the pack just in case. I had a MH subzero for years but it was always overkill, even with only a light sweater underneath. Legs basically a mid layer and shell. And as rgg said Bolivia's winter is awesome because it's mostly dry, the shell is there out of schizophrenic paranoia but that one time you don't have it a storm blows in.

You've got three wind-resistant/proof layers there, which is overkill and would have me, personally, drowning in sweat. A good base layer (with collar for sun protection) a light fleece midlayer then either a hardshell or soft-shell should be fine. Your synthetic insulated jacket goes on over the top at stops, hanging around camp or way up high if it's cold and windy.

If you put a synthetic insulated jacket, with nylon inner and outer shells, inside your hardshell jacket you put too many barriers for the sweat vapour to get through so it condenses before it reaches your 'breathable' hardshell and you get wet inside.

Given how dry Bolivia is (though this year is almost certainly an El Nino year i.e. wet storms) you don't really need a hard shell, but they're nice security.It also depends how windproof your soft-shell is but the TNF Apex is pretty windproof, iirc.

It gets quite cold up high in Bolivia, especially when you're starting in the dark hours. Something like an Odin or Pegasus is the minimum I would take, though you certainly don't need a big expedition down parka or similar. Make sure the hood fits over your helmet.

Bolivia is windy, so make sure to get a decent balaclava and/or face mask, such as one in the Outdoor Research line - windproof that covers your nose but allows your mouth to breathe.

It's a long time since I was there (July 1999) and I climbed in old style plastic boots. Leather/synthetic single boots are probably fine even on Illimani and would be nicer to climb in up from BC to Nido Condores C1 as that section was loose dangerous crappy rock and scree.

Have glove options - a light, durable, wind-resistant pair to wear on the approach, a warmer and burlier pair to actually climb in on the route, and a pair of warm mitts (not too heavy or fancy) for if it gets really cold or turns to shit.